There are two option to start your login:

Option #1:
In the address bar, add /admin at the end of the website/domain of the shop.
For example, if your shop is https://myshop.com , then enter: https://myshop.com/admin .

Option #2:
Or surf to: https://www.shopify.com/login

To see the full details of how to login:

See the next video (only from 0:15 to 1:05) … or read this official guide.